Fuel-injection system for engine and process for defining the beginning of pressure drop in common rail

ABSTRACT

A fuel-injection method and an apparatus provides an output timing of an injection command signal so that fuel-injection timing is matched with a basic desired injection timing, to reduce exhaust gases and engine performance, by filtering waveforms of the detected common rail pressure to obtain pressure data, calculating an approximate straight line from the pressure data, spanning from a preselected time to a time T 3  of at least the first smallest value after the start of the pressure drop in the common rail pressure, and defining a time, at which the difference between the pressure data and an approximate straight line Ld is the largest value, as the timing T 2  of the start of the pressure drop in the common rail pressure, then calculating the time lag ΔTd that spans from the output timing T o  of injection command signal to the fuel-injection timing

This a division of parent application Ser. No. 09/345,438, filed Jun.30, 1999, U.S. Pat. No. 6,227,168.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention-relates to a fuel-injection method and anapparatus therefor; in which fuel delivered through a common rail ischarged from injectors into combustion chambers, and further relates toa process for defining the beginning of pressure drop in the commonrail, which is applicable to the common-rail, fuel-injection system.

2. Description of the Prior Art

In a typical fuel-injection control for engines such as diesel engines,a common-rail, fuel-injection system is conventionally known, in whichthe fuel injected is highly intensified in pressure and thefuel-injection characteristics such as a timing and quantity of fuelinjected per cycle are adequately controlled in accordance with theengine operating conditions. On most common-rail, fuel-injectionsystems, the fuel pressurized at a preselected pressure is stored in afuel supply line common to the injectors and the consequent stored fuelis injected from each injector into the associated combustion chamber.In order to inject the pressurized fuel at the individual fuel injectorwith the optimal fuel delivery condition for the engine operation, acontroller unit regulates the fuel pressure in the common rail and thecontrol valves each installed in the individual fuel injector.

A conventional common-rail, fuel-injection system will be explainedbelow with reference to FIG. 7. The fuel delivery to injectors 1 iscarried out through a common rail 2 and injection lines 3, each of whichis a part of the fuel-flow line. A fuel feed pump 6 draws fuel from afuel tank 4 through a fuel filter 5 and forces it under a preselectedpressure to a fuel-supply pump 8 through a fuel line 7. The fuel-supplypump 8 is of, for example, a fuel-supply plunger pump driven by theengine, which intensifies the fuel to a high pressure determineddepending on the engine operating conditions, and supplies thepressurized fuel into the common rail 2 through another fuel line 9. Thefuel, thus supplied, is stored in the common rail 2 at the preselectedhigh pressure and forced to the injectors 1 from the common rail 2. Theinjectors 1 are provided in compliance with the type of engines (thenumber of the cylinders) and controlled with a controller unit 12 of anelectronic control unit to thereby inject the fuel, supplied from thecommon rail 2, to the associated combustion chambers with an adequatetiming and metered quantity of fuel. The injection pressure of the fuelsprayed out of the injectors 1 is substantially equal with the pressureof fuel stored in the common rail 2, that is, the common rail pressure,which is thus regulated in order to control the injection pressure.

The fuel relieved from the fuel-supply pump 8 is allowed to flow backthe fuel tank 4 through a fuel-return line 10. The unconsumed fuelremaining in each injector 1 out of the fuel fed through the injectionlines 3 into the injectors 1 may return to the fuel tank 4 through afuel-recovery line 11. The controller unit 12 is applied with varioussignals of sensors monitoring the engine operating conditions, such as acylinder identifying sensor and a crankshaft position sensor fordetecting the engine rpm Ne, an accelerator pedal sensor for detectingthe depression Acc of an accelerator pedal, an engine coolanttemperature sensor, an intake manifold pressure sensor and the like. Thecontroller unit 12 may regulate the fuel-injection characteristics, thatis, the injection timing and the quantity of fuel injected out of theinjectors 1, depending on the applied signals, to thereby allow theengine to operate as fuel-efficient as possible. Moreover, thecontroller unit 12 is applied with a detected single as to a common-railpressure reported from a pressure sensor 13 installed in the common rail2. Injection of fuel out of the injectors 1 consumes the fuel in thecommon rail 2, causing the pressure drop in the common-rail pressure, atwhich the controller unit 12 regulates the discharge of the fuel-supplypump 8 so as to keep the common-rail pressure constant.

As described in Japanese Patent Publication No. 60020/1985, the priorcommon-rail, fuel-injection system controls the fuel-injection pressureto the desired value in accordance with the engine operating conditions,while calculating the fuel-injection characteristics, that is, thequantity of fuel metered to be injected, which quantity is defined bythe pressure and duration for injection per cycle, and the timing offuel injection, in compliance with the engine operating conditions,thereby achieving the fuel-injection characteristics optimal for theengine operating conditions. The common-rail pressure defining theinjection pressure is intensified by the fuel-supply pump, whileregulated to a desired injection pressure by means of a pressureregulator valve.

In the prior common-rail, fuel-injection system, the controller unitapplies the command pulses for the injection command signals to thesolenoid-operated valves, which are provided in the injectors, each toeach injector. The solenoid-operated valves energized with the commandpulses lift the needle valves to open the injection holes at the nozzletips of the injectors 1, resulting in allowing the fuel charges into thecombustion chambers. However, a time-lag is usually present, spanningfrom the time when the controller unit issues the command pulse to besignaled to the solenoid-operated valve to the time when the fuel isactually injected out of the nozzle holes of the injectors. Such timelag arises from a response delay inherent in the driving circuit, thatis, a delay spanning from the time of signaling the command pulse fromthe controller unit to the solenoid to the time of the actualenergization of the solenoid, and a mechanical delay in the injectors,during which the needle valve is made to lift after the energization ofthe solenoid to thereby allow the fuel injection out of the injector.Moreover, even if the timing when the command pulses issued from thecontroller unit turn “on” is precisely maintained at constant, everyinjector has tended to vary or scatter in the timing of the beginning ofthe fuel injection owing to characteristic difference in the individualinjectors, aging or the like.

In most conventional fuel-injection systems dealing with the problem asdescribed just above, the time lag is considered to be constant so thatthe scattering for every injector in the time lag is ignored.Accordingly, the optimal combustion of fuel can not be appreciated dueto characteristic difference in the individual injectors, aging or thelike. This results in the major problems in which the exhaust gascontrol becomes inferior and vibration occurs in the engine owing to thedifference in combustion timing among the individual cylinders.

In contrast, disclosed in Japanese Patent Laid-Open No. 210174/1996 is amethod of detecting fuel-injection timing and an apparatus therefor,which has for its object to determine accurately the fuel-injectiontiming in the diesel engines. According to this prior art, the fuelpressure is monitored at a fuel line connecting a fuel injection pumpwith fuel-injection nozzles, while a pressure drop greater than thepreselected value is detected, which happens first after the monitoredfuel pressure reaches a design high pressure. The initiation of thefirst pressure drop is identified as the timing of the beginning of thefuel injection.

Unlike the common-rail fuel-injection system, nevertheless, the methodand apparatus for detecting fuel-injection timing, disclosed in theabove publication, belong to a fuel-injection system including a fueldistributor pump to meter and direct fuel to the injectors, or an inlinefuel injection pump.

On the other hand, Japanese Patent Laid-Open No. 47137/1998 disclosestherein a method of detecting fuel-injection timing and an apparatustherefor in a common-rail, fuel-injection system. A pressure sensor inthe common rail detects the timing when the pressure drop happens in thecommon rail pressure after the fuel injection out of the injectors. Theactual timing of the beginning of the fuel injection is calculated, incompliance with the timing of pressure drop, by going backwards by thelength of time during which the pressure waves are transmitted from theinjection nozzles to the common rail. The deviation of the resultantactual timing from the desired timing of the beginning of the fuelinjection is stored for compensating the desired timing of the beginningof the next fuel injection. That is to say, the above citation disclosesthe conception of compensating the desired timing of the start of thefuel injection with the detected timing of the start of pressure drop inthe common rail pressure.

However, since the pressure drop in the common rail pressure due to thefuel injection is normally accompanied by pulsative waves, it isactually very hard to detect accurately the timing of the beginning ofthe pressure drop in the common rail pressure. In this regard, theabove-cited Japanese Patent Laid-Open No. 47137/1998 discloses noteaching of the specific measures about how to detect the timing of thebeginning of the pressure drop of the common rail pressure.

Based on the recognition that defining closely the timing of thebeginning of pressure drop is critical for accurate control of thetiming of the initiation of the fuel injection on the individualinjectors in the common-rail, fuel-injection system, the inventors havealready proposed a method of defining accurately the timing of thebeginning of the pressure drop in the common rail pressure, which isexplained in co-pending senior Japanese Patent Laid-Open No.101149/1999, and further confirmed the results satisfactory to somedegree. Nevertheless, there is still the room for improvement and,therefore, how to define strictly the timing of the beginning ofpressure drop is the major subject for accurately controlling the timingof the initiation of the fuel injection on the individual injectors inthe common-rail, fuel-injection system.

SUMMARY OF THE INVENTION

A primary object of the present invention is to overcome the problems inthe prior art as having been described above, and to provide afuel-injection method and an apparatus therefor and further provide amethod of defining a timing of the beginning of pressure drop in acommon rail pressure, which is suitably applicable to the fuel-injectionmethod. The present invention is made by detecting the common railpressure with paying attention to the fact that the common rail pressurefalls in accordance with the quantity of fuel injected out of theinjectors on every fuel-injection, irrespective of the scattering on thefuel-injection timings of the individual injector, and finding withaccuracy the actual timing of the beginning of the fuel injection in theinjectors, depending on the timing at which the common rail pressurestarts to fall down.

The present invention is concerned with a fuel-injection method for anengine, comprising the steps of; storing a common rail with a fueldelivered by the means of a fuel-supply pump, injecting the fuel fromthe common rail into combustion chambers through discharge orificesformed in injectors that are actuated with injection command signals,making a decision about a basic desired injection timing in compliancewith engine operating conditions at present by using a basic desiredinjection timing data which is previously defined in accordance with theengine operating conditions, finding an injection lag spanning from anoutput timing of the injection command signal to an injection timing atwhich the fuel injection is made to start, by a function previouslydefined of a variable of a time at which a pressure drop starts in thecommon rail pressure at least after the start of the fuel injection, andmaking a decision about the output of the injection command signal inaccordance with the basic desired injection timing and the injectionlag.

The present invention is further concerned with a fuel-injectionapparatus for an engine, comprising; a common rail for storing therein afuel forced by the action of a fuel-supply pump, injectors havingdischarge orifices through which the fuel from the common rail isinjected into combustion chambers, means for monitoring engine operatingconditions, a pressure sensor for monitoring a pressure in the commonrail, and a controller unit for deciding a basic desired injectiontiming in compliance with the engine operating conditions at present byusing a basic desired injection timing data which is previously definedin accordance with the engine operating conditions detected with theengine condition monitoring means, and further for applying theinjectors with the injecting command signal in accordance with the basicdesired injection timing, wherein the controller unit finds an injectionlag spanning from the output timing of the injection command signal toan injection timing at which the fuel injection is made to start, by afunction previously defined of a variable of a time at which a pressuredrop starts in the common rail pressure at least after the start of thefuel injection, and makes a decision about the output of the injectioncommand signal in accordance with the basic desired injection timing andthe injection lag.

In one aspect of the present invention, a fuel-injection method isdisclosed, wherein the output timing of the injection command signal forthe recent fuel injection in the injectors is decided at a time goingbackwards by the injection lag found at the fuel injection last time inthe injectors from the basic desired injection timing.

In another aspect of the present invention, a fuel-injection method isdisclosed, wherein the timing of the start of the pressure drop in thecommon rail pressure is defined as a time at which a difference betweena pressure data and an approximate straight line is the largest value,the pressure data being obtained by filtering process of waveforms ofthe detected common rail pressure, and the approximate straight linebeing calculated, with respect to a curve represented on coordinates ofthe time and pressure data, by the use of the pressure data spanningfrom a preselected time before the pressure drop in the common railpressure to a time of at least the first smallest value after the startof the pressure drop.

In another aspect of the present invention, the start of pressure dropin the common rail pressure may be given by filtering the common railpressure through a low-pass filer to thereby obtain pressure data, andthen approximating a curve of pressure variation during the pressuredrop on coordinates of the time and the pressure data by the leastsquare method to thereby obtain an approximate straight line of thecurve during the pressure drop till the pressure data becomes the firstsmallest value. As a result, the start of pressure drop in the commonpressure is defined as the time on the time-axis at which the differencebetween the approximate straight line and the pressure data becomes thelargest value. According to the present invention, thus, the time of thetime-axis at which the difference between the pressure data becomes thelargest value, or the time at which the pressure data exceeds theapproximate straight line with the largest deviation during a length oftime till the first smallest after the start of the pressure drop in thecommon rail pressure, is regarded as the timing of the start of thepressure drop in the common rail pressure. Moreover, the timing of thestart of pressure drop in the common rail pressure defined as describedabove can be safely applied in practice for finding experimentally aninjection lag between the injection command timing and thefuel-injection timing.

According to another aspect of the present invention, the injection lagabout from the output timing of the injection command signal to thefuel-injection timing is individually calculated for each of theinjectors. The difference for every cylinder, or the difference indistance between the discharge orifice of the injector and the pressuresensor for the common rail pressure, may affect the length of timeduring which the pressure variation transmits. It will be understoodthat the difference for every cylinder included the individualscattering of the injectors, injection lines and the like.

The present invention is further concerned with a method of defining astart of pressure drop in a common rail pressure, comprised of the stepsof; filtering processing waveforms of the detected common rail pressureto thereby obtain pressure data, calculating an approximate straightline of a curve of pressure data variation on coordinates of the timeand pressure data by making use of the pressure data spanning from apreselected time before the pressure drop to a time of at least thefirst smallest value after the start of the pressure drop, and defininga time, at which a difference between a pressure data and an approximatestraight line is the largest, as the timing of the start of the pressuredrop in the common rail pressure.

It is preferred that the preselected time before the pressure drop isthe timing of issuing the fuel-injection command signal. Moreover, thecommon rail pressure may be detected based on the output timing of theinjection command signal or may be detected dependent on the degree ofstability on the common rail pressure before signaling of the commandpulse. The approximate straight line may be calculated by the use of theleast square method. In either case, the present invention makes itpossible to define accurately the timing of the start of pressure drop.

As apparent from the foregoing description, the present inventiondevelops to find the injection lag between the output timing of theinjection command signal and the fuel-injection timing by a previouslydefined function, depending on the start of pressure drop in the commonrail pressure after the fuel-injection, thereby making a decision aboutthe output timing of the subsequent injection command signal inaccordance with both the basic desired injection timing and theinjection lag. That is to say, the length of time spanning from theoutput of the injection command signal to the start of the actual fuelinjection, or the injection lag, may be found by the previously definedfunction, dependent on the timing of the start of pressure drop in thecommon rail pressure, which is calculated based on the pressure data ofthe common rail pressure detected at the pressure sensor. The outputtiming of the injection command signal to be applied to the injectors isdefined based both of the injection lag found as described above and thebasic desired injection timing found as the optimal injection timing inaccordance with the engine operating conditions such as acceleratorpedal depression and the engine rpm or the like. Accordingly, the actualfuel-injection may begin at the basic desired injection timing designedpreviously so as to help ensure the most suitable exhaust gases andengine output performances, irrespective of variations in the injectorssuch as aging in the response to the injection command signals and thescattering in fuel-injection timing for every injector, therebyresulting in improving the exhaust gases and engine output performances.

In contrast, on the prior fuel-injection system in which the injectionlag between the output timing of the fuel-injection command signal andthe actual fuel-injection timing is considered to be constant withoutpaying attention to the scattering in the injection lag, even if theinjection command signals have been initially designed so as to signalat the optimal timing, it may be possible that the start of thefuel-injection becomes out of the optimal timing owing to the aging inthe injection lag and, in addition, the individual injector has usuallythe scattering or deviation in the start timing of the fuel-injection.For the reasons as described above, the vibration and inferior exhaustgas control have occurred in the prior engines owing to the scatteringin the timing of the combustion sequentially carried out in theindividual cylinder of an engine, to say nothing of the individualengine. Nevertheless, the fuel-injection method the apparatus accordingto the present invention makes it possible to achieve the actual fuelinjection at the optimal basic desired injection timing, resulting inpreventing the vibration and inferior exhaust gas control.

Moreover, according another aspect of the present invention, the startof the fuel injection may be decided by processing the informationdetected at the common rail pressure sensor that is conventionallyincorporated in the common-rail system and, therefore, there is no needof developing a novel pressure sensor for making a decision about thestart timing of the fuel injection. Unlike the fuel-injection systemhaving the distributor injection pump or inline fuel injection pump, thenumber of pressure sensors is not necessarily the same as the number ofinjection lines connecting the fuel injection pump with the injectors,so that no increase of the number of the parts may be necessary, withresulting in cost saving. Moreover, detecting the common rail pressurein the common-rail, fuel-injection system is more concurrentlysusceptible to the large changes in the common rail fuel pressure due tothe engine operating conditions, compared with another prior system inwhich the information effective for detecting is obtained by comparingthe pressure in the injection lines connecting the fuel-injection pumpand the injectors to the a design pressure threshold.

The fuel-injection method and apparatus therefor of the presentinvention adopt the method of defining the start of pressure drop in thecommon rail pressure comprised of the steps of calculating anapproximate straight line of a curve of the pressure variation of thecommon rail pressure after filtering by making use of the pressure datatill a time of at least the first smallest value on the curve, anddefining a time, at which a difference between the pressure data and theapproximate straight line is the largest, as the timing of the start ofthe pressure drop in the common rail pressure. This makes the load onthe DSP lighter and the influence on the quantity of fuel charge morereduced, compared with the senior co-pending application in Japan.

Other objects and features of the present invention will be moreapparent to those skilled in the art on consideration of theaccompanying drawings and following specification wherein are disclosedpreferred embodiments of the invention with the understanding that suchvariations, modifications and elimination of parts may be made thereinas fall within the scope of the appended claims without departing fromthe spirit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a composite graph of several variable, that is, injectioncommand signal or command pulse, injection rating and common railpressure, versus time in a common rail, fuel-injection system:

FIG. 2 is a flowchart illustrating a main processing procedure to beexecuted in a fuel injection according to the present invention:

FIG. 3 is a flowchart illustrating an interrupt processing procedure ofa cylinder-identifying signal in the execution of the procedure shown inFIG. 2:

FIG. 4 is a flowchart illustrating a main processing procedure in adigital signal processor, referred to as DSP hereinafter, forcalculating an injection lag in the execution of the procedure shown inFIG. 2:

FIG. 5 is a flowchart illustrating a 100 kHz interrupt processingprocedure for buffering the data of the common rail pressure tocalculate the injection lag in the execution along the flowchart in FIG.3:

FIG. 6 is a flowchart illustrating a procedure for calculating theinjection lag, basing on the buffered data of common rail pressure bythe execution along the flowchart in FIG. 5: and

FIG. 7 is a schematic illustration of an exemplary common-rail,fuel-injection system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a fuel-injection system according to thepresent invention will be explained in detail hereinafter with referenceto the accompanying drawings.

A fuel-injection method and an apparatus therefor in internal combustionengines are suitable for use in the common-rail, fuel-injection systemshown in FIG. 7. Most of components of the system, thus, are the same aspreviously described. To that extent, the components have been given thesame reference characters as shown in FIG. 7, so that the previousdescription will be applicable.

First explaining the relations in the injection command signal orcommand pulse, injection rating and common rail pressure, versus timewith reference to the composite graph shown in FIG. 1. Now assuming thata command pulse is turned “ON” at a time T₀, or the output timing of anfuel-injection command signal, before the top dead center of theindividual cylinder n, the fuel is injected out of the injector 1 at atime T₁ for fuel injection after a time interval or time lag ΔTd. Anactual common rail pressure Pf in the common rail 2 does not start tofall just after the start of fuel injection, but start to fall with asomewhat time lag. When detecting the pressure drop of the actualpressure Pf in the common rail 2 at a pressure sensor 13, thefuel-supply pump 8 is energized to force the fuel into the common rail 2thereby restoring the pressure in the common rail 2. A command value ΔTbfor a desired fuel-injection timing is found, which goes backwardsbefore the top dead center in compliance with the engine operatingconditions. A basic desired fuel-injection timing Tb is determined andfurther the time lag ΔTd is found depending on the timing of the startof pressure drop in the common rail 2, whereby the output timing T₀ offuel-injection command signal, or command pulse, is determined at thetime going backwards by the time lag ΔTd from the basic desiredfuel-injection timing Tb.

The command value ΔTb for the desired fuel-injection timing is a commandvalue for defining the basic desired fuel-injection timing Tb at whichthe fuel injection is to be actually initiated, and found as the timeinterval going backwards from the top dead center. The command value ΔTbfor the desired fuel-injection timing is previously mapped in accordancewith the engine operating conditions. Under the events where thefuel-injection timing is controlled properly, the time T₁ of fuelinjection is in coincidence with the basic desired fuel-injection timingTb. A command value ΔTc for fuel-injection timing is the output timingT₀ of fuel-injection command signal for the injectors 1, and found asthe time interval going backwards from the top dead center. Moreover,the time lag ΔTd of fuel injection is a time interval or time delaybetween the output of the fuel-injection command signal and the start ofactual fuel injection and, therefore, be as specified for the individualinjector. The time lag ΔTd of fuel injection, when emphasizing thedifference for every individual injector, will be hereinafter expressedby ΔTd(n) to identify the associated cylinder number n.

The main processing procedure of the fuel injection will be describedbelow in conjunction with the flowchart in FIG. 2. It is to be notedthat the controller unit for managing the fuel injection of the enginemain is enabled for interruption in the main processing procedure at thenecessary time interval.

Calculating the engine rpm Ne by a tachometer on the output shaft of theengine, which monitors pulses occurring in accordance with the enginerpm (Step 1, referred to as S1 and so forth).

Calculating the accelerator pedal depression Acc in accordance withsignals issued from the accelerator pedal sensor (S2).

Calculating the basic quantity of fuel charge in accordance with theengine rpm Ne given at S1 and the accelerator pedal depression Acc givenat S2, in comparison with lookup tables in which are tuned up thepreviously-stored data about exhaust gases, engine output andcomfortable drive-feeling (S3).

The command value ΔTb for a desired fuel-injection timing is found,which goes backwards before the top dead center in compliance with theengine rpm Ne given at S1 and the accelerator pedal depression Acc givenat S2, and the basic desired fuel-injection timing Tb is calculated(S4). As an alternative, the basic quantity of fuel charge obtained atS3 may be employed instead of the accelerator pedal depression Acc givenat S2.

Calculating the common-rail pressure Pf, or the actual pressure in thecommon rail 2, in accordance with the signals issued from the commonrail pressure sensor 13 (S5).

Calculating the desired pressure in the common rail 2 sufficient toobtain the basic quantity of fuel charge in accordance with the enginerpm Ne at some preselected time, or calculating the desired common railpressure Pf₀, by making use of both the engine rpm Ne given at S1 andthe basic quantity of fuel charge given at S3 (S6).

The Controller unit 12 regulates the variable stroke, fuel-supply pump 8such that the common-rail pressure Pf, obtained at S5 is brought intocoincidence with the desired common rail pressure Pf₀ calculated at S6(S7).

The processing procedure described above is executed repeatedly wheneverthe engine is operated.

The following in conjunction with the flowchart in FIG. 3 is for theinterrupt processing procedure of the cylinder-identifying signals. Thecylinder number is expressed by n.

Calculating the command value ΔTc for fuel-injection timing, given bythe following formula;

ΔTc=ΔTb+ΔTd(n)

where ΔTb is the command value for a desired fuel-injection timing foundat the above S4 and ΔTd(n) represents the time lag of fuel injection forthe cylinder n, which is found along a flowchart described hereinafter(S10). Thus, the ΔTd(n) is a positive value that may differ for everycylinder. By adding the command value ΔTb for a desired fuel-injectiontiming, which is found in accordance with the engine rpm Ne and theaccelerator pedal depression Acc and determined as a length of timegoing backwards before the top dead center, and the ΔTd(n) found asdescribed hereinafter, the output timing T₀ of an fuel-injection commandsignal issued from the controller unit 12 is determined at a timing thatgoes backwards by the command value ΔTc for fuel-injection timing fromthe time of the top dead center.

The quantity of fuel charge is corrected in compliance with therecalculated fuel-injection timing (S11).

The corrected quantity of fuel charge is converted to a pulse width ofcommand pulse for the fuel-injection command signal (S12). In otherwords, the pulse width of the command pulse applied to the solenoids inthe injectors 1 is determined, which pulse width defines a durationduring which the solenoid-operated valves are kept open to inject thecorrected quantity of fuel.

Actual injection of fuel is executed and the timing and the pulse widthof the command pulse are stored in the output register (S13).

When the cylinder-identifying signal is detected for the individualcylinder n at the preselected time before the top dead center of theexplosion phase or power stroke, the interrupt processing proceduredescribed just above is executed and, after the fuel injectionterminates, the main procedure shown in FIG. 2 is resumed.

The following in conjunction with the flowchart in FIG. 4 is for themain processing procedure in the digital signal processor, abbreviatedto DSP, for calculating the fuel-injection lag ΔTd(n) described above inS10. The DSP is adopted in this system for parallel processing with CPUbecause the calculation of the fuel-injection lag requires buffering thevast data and processing the data at high speed in a short time.Nevertheless, the parallel processing by the use of DSP is notnecessarily required, provided that the CPU has the sufficient abilityof high-speed processing.

Variables to be processed are initialized (S20).

DSP is initialized and the interrupt processing is set up (S21).

Whether the data required for calculating the fuel-injection lag ΔTd(n)is buffered or not is decided (S22). This decision might be based on thestate of a data-buffering ending flag Flag 2. The flag 2 is set up onthe 100 KHz interrupt processing described below. With the buffering inending, the procedure advances to the next step 23. In contrast, whenthe buffering is not yet in ending, the procedure is made to wait.

Calculating the fuel-injection lag ΔTd(n) (S23). The details ofcalculating steps will be explained in conjunction with the flowchartsin FIGS. 5 and 6.

The data-buffering ending flag Flag 2 is cleared to get ready for thestorage of data required for the next fuel-injection lag ΔTd(n) (S24).

Next, the 100 KHz interrupt processing procedure will be explained withreference to FIG. 5. For calculating the fuel-injection lag ΔTd(n),values of the common rail pressure Pf are buffered with the cycle of 100KHz. The buffering is initiated on the timing at which the command pulseto the injectors 1 starts to rise, or turned “ON”. The ending of thebuffering is preselected so as provide any time enough for confirmingthe pressure drop and pressure wave in the common rail pressure Pf. Thatis to say, it is predetermined experimentally with consideration for theinjection lag and the somewhat scattering in the common rail pressure.

Whether a flag Flag 1 showing the state under the data buffering is setor not is decided (S30).

Under the data buffering the procedure advances to the step 34,otherwise advances to step 31.

In the absence of data buffering, the pulse edge of command pulse toenergize the solenoids in the injectors 1 is detected (S31). Thepresence of the command pulse edge advances the procedure to the step32, otherwise the 100 KHz interrupt processing terminates.

Based on the detected command pulse edge or the detection of theinjection command signal issued, the flag Flag 1 is set (S32).

Identifying the number of the cylinder provided with the injectorapplied with the command pulse at that time (S33). The cylinder number nis stored in a memory at the interrupt processing procedure ofcylinder-identifying signal.

The common rail pressure Pf is buffered as the data [Data(i)], afterhaving set the flag Flag 1 during data buffering or having detected thecommand pulse edge (S34).

Deciding as to whether the acquired number of the data(i) as to thecommon rail pressure Pf is more than or equal to the desired amount Ts(S35). In case where the acquired number of the data i does not reachthe desired amount Ts, the procedure returns to S30 to continue theacquisition of the data. In contrast, with the acquired number of thedata i reaching the desired amount Ts, the buffering processingterminates and the procedure advances to S36. Since the scattering ofthe fuel-injection lag ΔTd(n) is limited in a certain range, the desiredamount Ts may be sufficient, that is about several times the largestvalue of the fuel-injection lag ΔTd(n).

The flag Flag 1 is cleared (S36).

The buffering counter i for the number of data is cleared (S37).

The data buffering ending flag Flag 2 is set (S38).

Finally, referring to FIG. 6, the following will explain the calculationof the fuel-injection lag ΔTd(n) on the buffered data of the common railpressure Pf.

Filtering the data (S40). The data of the common rail pressure Pfdetected at the pressure sensor 13 contains usually noise and,therefore, the data is subjected to the filtering processing with alow-pass filter or the like.

Differentiation of the common rail pressure Pf is executed with respectto the time to find the pressure variation on the common rail pressurePf (S41). As the acquired data about the common rail pressure Pf is thediscrete data with respect to the time, the differential calculation maybe carried out with the finite differences between any adjoining datavalues.

Finding a time T₃ at which the common rail pressure Pf becomes the firstsmallest value after the start of the pressure drop thereof, because theacquired population of the data is initiated after the rise of thecommand pulse has been detected (S42).

Calculating an approximate straight line Ld, shown with a broken line inFIG. 1, of the common rail pressure data on a length of time from thetime To of the rise of the command pulse to the time T₃, by the primaryregression equation (S43). That is to say, the approximate straight lineLd of the pressure drop curve on the common rail pressure Pf iscalculated with respect to the common rail pressure variation spanningfrom the time T₀ of the rise of the command pulse to the time T₃ by theuse of the least square method. Calculating a time P₁ at which thedifference between the approximate straight line Ld and the common railpressure Pf after filtering becomes the largest value. The time P₁ isrecognized as the timing T₂ of the start of the pressure drop in thecommon rail pressure (S44).

Calculating the injection lag ΔTd(n) resulting from the formula

ΔTd(n)=f(T ₂ , Pf, n)  (S 45)

namely, the injection lag ΔTd(n) is denoted by the function of the threevariables, or the time T₂ at P₁ where the difference between theapproximate straight line Ld and the common rail pressure Pf, is thelargest value, the common rail pressure Pf and the cylinder number n,and therefore may be calculated by assigning the values, found actuallyat the above-described steps, to the associated variables in thefunction f. The start of the fuel-injection has tendency to retard asthe time T₂ becomes later. Moreover, the injection lag ΔTd(n) is relatedto how far the cylinder number n is from the pressure sensor, namely,further the cylinder is from the common-rail pressure sensor, longer isthe injection lag ΔTd(n). Besides the time T₂ and cylinder number n, itmay be considered that the injection lag ΔTd(n) is affected by thedifference in the transmitting speeds of the pressure waves owing to thedifference in magnitude of the common rail pressure Pf before the startof the pressure drop in the common rail pressure. Now, assuming thatthere is noor little influence of the difference in magnitude of thecommon rail pressure Pf, the injection lag ΔTd(n) may be calculated bythe formula

ΔTd(n)=f(T ₂ , n)

Although the command pulse, fuel-injection rating and common railpressure have been explained with respect to the lapse of time in theembodiments described above, other parameter such as crank angle may beused as long as showing substantially the lapse of time.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description proceeding them, and all changes that fall within meetsand bounds of the claims, or equivalence of such meets and bounds aretherefore intended to embraced by the claims.

What is claimed is:
 1. A method of defining a start of pressure drop ina common rail pressure, comprised of the steps of; filtering processingwaveforms of the detected common rail pressure to thereby obtainpressure data, calculating an approximate straight line of a curve ofpressure data variation on coordinates of the time and pressure data bymaking use of the pressure data spanning from a preselected time beforethe pressure drop to a time of at least the first smallest value afterthe start of the pressure drop, and defining a time, at which adifference between the pressure data and the approximate straight lineis the largest value, as the timing of the start of the pressure drop inthe common rail pressure.